Method for the electrolytical deposition of highly ductile copper

ABSTRACT

A homogenous, brittle-free, pore-free and dense copper layer of high ductility and having a content of included gases and/or organic impurities of less than 100 ppm is electrolytically deposited on electrically conducting surfaces from a bath comprising the copper ion and the pyrophosphate ion by maintaining the electrolytic bath constantly free from particles having a size substantially exceeding 3 microns.

United States Patent Fluhmann et al.

54] METHOD FOR THE ELECTROLYTICAL DEPOSITION OF HIGHLY DUCTILE COPPER [72] Inventors: Werner Fluhmann, Zurich; Walter Saxer,

Urdorf, Zurich, both of Switzerland [73] Assignee: Werner Fluhmann and Galvanische, Zu-

rich, Switzerland [22] Filed: July 6, 1970 [21] Appl. No.: 52,730

[30] Foreign Application Priority Data July 10, 1969 Switzerland ..10563/69 [52] US. Cl. ..204/15, 204/52 R, 204/238, 204/240, 210/24, 210/39 [51] Int. Cl ..C23b 5/48, C23b 5/18, BOlk 3/00 [58] Field of Search ..204/276, 240, 238, 15; 210/24, 210/39 [56] References Cited UNITED STATES PATENTS 1,798,994 3/1931 Whalin ..204/238 45] May2, 1972 Primary Examiner-John H. Mack Assistant Examiner-T. Tufariello Attorney-Werner W. Kleeman ABSTRACT A homogenous, brittle-free, pore-free and dense copper layer of high ductility and having a content of included gases and/or organic impurities of less than 100 ppm is electrolytically deposited on electrically conducting surfaces from a bath comprising the copper ion and the pyrophosphate ion by maintaining the electrolytic bath constantly free from particles having a size substantially exceeding 3 microns.

14 Claims, No Drawings METHOD FOR THE ELECTROLYTICAL DEPOSITION OF HIGHLY DUCTILE COPPER This invention is concerned with improvements in or relating to the production of copper coatings upon the electrically conducting surface of materials. It relates particularly to the electrolytical deposition of coppercoatings having improved qualities, and is mainly concerned with the preparation of socalled printed circuits.

Several methods for the electrodeposition of metals, e.g. of silver, gold, nickel, chronium, zinc, copper etc., are well known to the one skilled in the art. The general procedure is as follows:

The article to be electrolytically coated is immersed in an electrolyte bath containing ions of the metal to be deposited. This electrolyte may further contain organic and/or inorganic additives capable of influencing the metal coating during its deposition, the electric conductivity, the current density etc.. The article is then connected as a cathode, and electrolytic (galvanic)deposition is effected by direct current. This current may be interrupted from time to time and/or reversed for short periods of time in'order to modify the coating thickness. Temperatures, pH values and agitation of the bath are controlled.

When the desired thickness of the coating is achieved, calculable from amperage, current flow time and electrode surface, the article is withdrawn from the bath, thoroughly rinsed, and dried.

Nonmetallic articles may be made electrically conductive on the surface by one of the known techniques; see, e.g., K. Heymann et al. in "Angewandte Chemie," Vol. 82(l970), pages 412-421.

It is known from experience that metallic coatings made by electro-deposition exhibit higher hardness and lower ductility than coatings produced by other metallurgical methods. This is a disadvantage, since for many technical applications the ductility of the material represents a main requirement and since only ductile coatings warrant the prevention of fissuration. The two low ductility is generally caused by organic inclusions which are inevitably co-deposited during the electrolytic deposition from electrolytic baths containing brightening agents or other organic substances.

Likewise, metallic coatings produced by electro-deposition contain a higher quantity of gas than metals produced by metallurgical means. This disadvantage appears especially at soldering, since heating liberates the enclosed gas, by which the soldering process is hampered.

Furthermore, by organic inclusions and high gas content the electrical and thermal conductivity of metallic coatings are decreased.

These problems and the accompanying disadvantages are also known from electroplating copper. It is an object of this invention to provide a method which eliminates the aforementioned disadvantages by using an electrolytic bath which owing to its composition,mode of preparation and by the choice of the process parameters renders the best possible conditions. By a high degree of purity of the electrolyte it is sought to obtain ductile, bright, dense and pore-free copper electrodeposits of high purity.

Attempts have already been made to produce high ductile, hard, brittle-free and pure copper platings, however, these attempts were not successful.

In the electrodeposition of copper according to known methods, copper coatings of low ductility are obtained. The expression low ductility as used herein means a ductility of l to 6 generally 1 to 3 measured as ultimate linear elon gation at normal temperature. The expression high ductility" as used herein and as achieved by the present method, means ductilitiesof more than 6 generally 10 to 30%, particularly to It is important that this high ductility is obtained without proportional decrease in ultimate tensile strength and hardness.

lt-is' therefore theprincipal object ot this invention to provide a method for producing high ductile, dense andpore-free copper electrodeposits of high purity having a bright surface and having a content of organic matter and/or gases of less than ppm. This object of the invention is accomplished by the present method which comprises the use of an extremely pure electrolytic deposition bath containing at least one member comprising copper pyrophosphate and the combination of copper sulfate and potassium pyrophosphate, said electrolyte being free from particles having a size greater than about 3 microns.

The method of the invention supplies copper deposits of high ductility, Le. 10 to 30% ultimate elongation, having an ultimate tensile strength of about 27 to 29 kg/mm, under a load of 50 g (KHN A more outstanding feature is the excellent toughness of the coatings of the invention. Toughness is to be understood as the product of ultimate tensile strength and ductility (ultimate elongation), and the coatings of the inventions have been found to have toughnesses of about 400 to 650 kg/mm, particularly about 5 60 kg/mm.

It is an important feature of the invention to use extremely pure electrolytic baths. The ingredients used in conventional galvanotechnique to make up the deposition baths are not pure enough to be suited for the method of the invention since they always contain organic matter and/or insoluble particles geater than 3 microns.

Furthermore, dust from the surrounding atmosphere may fall into the bath. It is only the consequent introduction of extreme purity, known only from certain branches of the pharmaceutic industry, into the present method which conducted to its outstanding results.

The content of organic matter and/or gases included in the coatings of the invention is measured by vacuum extraction at 1,300 C. in high vacuum. Both organic matter and gases, the former by its pyrolysis products in gaseous form, may be assayed according to this method.

The method according to the invention is especially applicable for the electrodeposition of copper on plastic materials and on printed circuits in electrotechniques. Because of its high ductility and since it does not fissurate during compression and elongation, the deposited copper can be pressed level with the matrix material. In order to continuously maintain the conditions of the process according to the invention, it is not only necessary to start with an electrolyte which is free of particles over 3 pm in size, but which must be continuously cleaned during the deposition. This is best carried out by a circulation whereby the. electrolyte is purified outside the bath. The electrolyte can either be filtered through a microfilter or first purified with an adsorption agent and then filtered through a microfilter. The microfilters have preferably a pore diameter between 0.1 and 1.5 pm, in particular 0.5 gm; therefore the use of molecular sieves is recommended. Adsorption agents which are suitable are active carbon, silicagel or alumina. During the copper deposition the electrolyte should preferably have a temperature between 30 and 75 C, in particular 55 C. A pH-value of the electrolyte between 7 and 9, in particular of 8, is indicated.

In the course of the investigations it appeared that periodically changing the polarity or periodically interrupting the electrolysis current favorably influenced a homogeneous deposition of the copper. The same efiect occurs when working with an alternating current superimposed on the electrolysis current.

Before the eiectrolytic copper deposits can be applied on printed circuits, these have to be clad with copper either on one side or on both sides, or, the printed circuits can also be built up according to the multi-layer technique. in order to achieve a homogeneous deposition of copper on the walls of the holes of the printed circuits, it is recommended that these be previously made conductive, for example by treatment with chemical agents. The following examples should illustrate more specifically the process according to the invention and its application, but are not to be construed as limiting the invention in any aspect.

Physical properties of deposited copper coatings EXAMPLE 1 Printed circuits clad on both sides and with a plate thickness of 1.6 mm which are provided with holes of 0.4 mm in diameter are made electrically conductive by means of one of the 5 known methods. For the electrolytical reinforcement of the chemically deposited copper layer a bath of the following composition is used under operating conditions as stated hereafter:

25 g/l copper in form of copper pyrophosphate 200 g/l potassium pyrophosphate pH value 8 The electrolyte is first purified over active carbon and afterwards filtered in three successive steps through microfilters, i.e. through filters with pore diameters of 3 pm, l.2 um, and 0.6 pm respectively.

This bath was operated by switching the polarity in cycles of 10 seconds cathodically and 1 second anodically at a cathodic current density of 2 Amp/dm and an anodic current density of l Amp/dm The bath temperature was 55 C. It was continuously filtered through a microfilter, pore size 0.8 pm and in addition agitated by moving the cathode shaft.

One obtained a ductile, bud free and bright copper layer on the printed circuits, whereby the layer thickness of the copper deposit of 40 um was the same on the surface of the plate as well as on the walls of the holes, inclusive of the edges of the latter.

EXAMPLE 2 A bath which contains g/l copper in form of copper 3o pyrophosphate and 250 g/l potassium pyrophosphate is purified over active carbon and then filtered in steps through microfilters of decreasing pore size diameter. The filtration is then continued at a pore size diameter of 0.1 pm until no residues can be detected on the filter under the light microscope.

If during the copper deposition the purification over active carbon as well as the filtration over microfilters is maintained, one obtains copper deposits whose ductility is at least 20% and whose gas content does not exceed 1 ppm N 6 ppm H and 50 ppm 0 respectively. Such deposits are of extremely fine grain size, they are dense and free of pores and used preferably as a raw material in ultra-high vacuum technology at elevated temperatures.

The properties of the coatings of Example 1 and 2 are compiled in the following table.

TABLE We claim: 1. In a method for the electrolytrc deposition of pore-free metallic copper of high ductility and purity, especially for the deposition of copper coatings at printed circuits, containing at least less than ppm of one of the ingredients comprising organic matter and gaseous inclusions, from an electrolytic bath containing at least one composition of the group consisting of copper pyrophosphate and the combination of cop er sulfate and potassium pyrophosphate, the improvement comprising the steps of continuously initially passing the electrolytic bath through an adsorption agent to purify the electrolytic bath and then filtering the electrolytic bath in stages by successively passing such electrolytic bath through microfilters of decreasing pore size, the pore size of the last microfilter at most amounting to L5 microns.

2. The method as defined in claim 1, including the step of using as the microfilters for filtering the electrolytic bath micropore filters.

3. The method as defined in claim 2, including the step of utilizing micropore filters having a pore size in a range between 0.1 and 1.5 microns, and wherein the last micropore filter through which the electrolytic bath passes has a pore size which does not exceed 0.8 microns.

4. The method as defined in claim 2, including the step of using micropores filters having a pore size of approximately 0.5 microns.

5. The method as defined in claim 1, including the step of using as the adsorption agent a molecular sieve.

6. The method as defined in claim 1, including the step of using as the adsorption agent at least one member selected from activated carbon, silicagel or alumina.

7. The method as defined in claim 1, including the step of maintaining the electrolytic bath at a temperature which is in a range between approximately 33 C and 75 C.

8. The method as defined in claim 7, including the step of using an electrolytic bath possessing a temperature of approximately 55 C.

9. The method as defined in claim 1, including the step of controlling the pH-value of the electrolytic bath so as to be in a range between 7 and 9.

10. The method as defined in claim 9, including the step of controlling the pH-value of the electrolytic bath so as to possess a value of approximately 8.

11. The method as defined in claim 1, including the step of utilizing an electrolysis current for the electrolytic deposition wherein the polarity of said electrolysis current is periodically changed. 7

12. The method as defined in claim 1, including the step of utilizing an electrolysis current for the electrolytic deposition wherein the electrolysis current is periodically interrupted.

13. The method as defined in claim 1, including the step of utilizing an electrolysis current for the electrolytic deposition and superimposing an alternating current upon said electrolysis current.

14. In a method for the electrolytic deposition of pore-free metallic copper of high ductility and purity, containing at least less than 100 ppm of one of the ingredients comprising organic matter and gaseous inclusions, from an electrolytic bath containing at least copper ions and pyrophosphate ions, the improvement comprising the steps of continuously initially passing the electrolytic bath through an adsorption agent to purify the electrolytic bath and then stepwise continuously filtering the electrolytic bath by successively passing such electrolytic bath through a number of non-ceramic microfilters of progressively decreasing pore size, the pore size of the last microfilter at most amounting to 1.5 microns, the electrolytic bath thus being freed of particles of a size exceeding 3 microns.

i K k t i 

2. The method as defined in claim 1, including the step of using as the microfilters for filtering the electrolytic bath micropore filters.
 3. The method as defined in claim 2, including the step of utilizing micropore filters having a pore size in a range between 0.1 and 1.5 microns, and wherein the last micropore filter through which the electrolytic bath passes has a pore size which does not exceed 0.8 microns.
 4. The method as defined in claim 2, including the step of using micropores filters having a pore size of approximately 0.5 microns.
 5. The method as defined in claim 1, including the step of using as the adsorption agent a molecular sieve.
 6. The method as defined in claim 1, including the step of using as the adsorption agent at least one member selected from activated carbon, silicagel or alumina.
 7. The method as defined in claim 1, including the step of maintaining the electrolytic bath at a temperature which is in a range between approximately 33* C and 75* C.
 8. The method as defined in claim 7, including the step of using an electrolytic bath possessing a temperature of approximately 55* C.
 9. The method as defined in claim 1, including the step of controlling the pH-value of the electrolytic bath so as to be in a range between 7 and
 9. 10. The method as defined in claim 9, including the step of controlling the pH-value of the electrolytic bath so as to possess a value of approximately
 8. 11. The method as defined in claim 1, including the step of utilizing an electrolysis current for the electrolytic deposition wherein the polarity of said electrolysis current is periodically changed.
 12. The method as defined in claim 1, including the step of utilizing an electrolysis current for the electrolytic deposition wherein the electrolysis current is periodically interrupted.
 13. The method as defined in claim 1, including the step of utilizing an electrolysis current for the electrolytic deposition and superimposing an alternating current upon said electrolysis current.
 14. In a method for the electrolytic deposition of pore-free metallic copper of high ductility and purity, containing at least less than 100 ppm of one of the ingredients comprising organic matter and gaseoUs inclusions, from an electrolytic bath containing at least copper ions and pyrophosphate ions, the improvement comprising the steps of continuously initially passing the electrolytic bath through an adsorption agent to purify the electrolytic bath and then stepwise continuously filtering the electrolytic bath by successively passing such electrolytic bath through a number of non-ceramic microfilters of progressively decreasing pore size, the pore size of the last microfilter at most amounting to 1.5 microns, the electrolytic bath thus being freed of particles of a size exceeding 3 microns. 